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Unusual Reaction of 1 1-Dilithio-2 3 4 5-tetraphenylsilole with 1 3-Dienes Yielding Spirosilanes and Elemental Lithium.

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Communications
Silanes
DOI: 10.1002/anie.200503362
Unusual Reaction of 1,1-Dilithio-2,3,4,5tetraphenylsilole with 1,3-Dienes Yielding
Spirosilanes and Elemental Lithium**
Irina S. Toulokhonova, Dennis R. Friedrichsen,
Nicholas J. Hill, Thomas Mller, and Robert West*
Silole dianions[1–6] are important species in organometallic
chemistry, both because of their high reactivity and their
unique electronic structure. Silole dianions possess six
[*] Dr. I. S. Toulokhonova, D. R. Friedrichsen, Dr. N. J. Hill,
Prof. Dr. R. West
Department of Chemistry
University of Wisconsin, Madison
1101 University Ave., Madison, WI 53706 (USA)
Fax: (+ 1) 608-262-6143
E-mail: west@chem.wisc.edu
Dr. T. M;ller
Institut f;r Anorganische und Analytische Chemie der Universit=t
Frankfurt
Marie-Curie-Strasse 11, 60439 Frankfurt (Germany)
Fax: (+ 49) 69-798-29188
[**] This research was supported by grants from the National Science
Foundation. We thank Dr. Ilia A. Guzei (UW-Madison) for solving
and refining the twinned crystal structure of 5.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2578 –2581
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Chemie
p electrons, and it is well established from theoretical
calculations[2] and X-ray data that silole dianions (A)[3–6] and
their benzannulated analogues (B, C)[7–10] have aromatic
character. Silole dianions act as nucleophiles in reactions
with halogenated alkyl groups, silanes,[1, 4–12] and adamantanone,[13] and have strong reducing properties.[14]
dilithio-9-silafluorene (4) and DMB to yield spirosilafluorene
5, also in high yield (Scheme 2).
Scheme 2. Reaction of silafluorene dianion 4 with DMB.
Reactions with 1,3-dienes to yield 1,4-cyclic products are
characteristic of silylenes.[15] Indeed, compound 2 was first
reported from the reaction of photochemically generated
silylene 6 with DMB.[16] Thus, we considered that the reactions
in Scheme 1 might proceed through the same silylene 6. No
Here, we report the unexpected reaction of 1,1-dilithio2,3,4,5-tetraphenylsilole (1) with 2,3-dimethyl-1,3-butadiene
(DMB) and 1,3-cyclohexadiene to yield the spirosilole
products 2 and 3 (Scheme 1). Remarkably, the lithium ions
Scheme 1. Reaction of silole dianion 1 with DMB and with 1,3-cyclohexadiene.
are reduced in these reactions to metallic lithium. The
elemental lithium appeared as a gray powder at the bottom
of the reaction vessel and was identified by reaction with 2propanol to produce dihydrogen, by interaction with benzyl
chloride to give 1,2-diphenylethane, and by titration of a
hydrolyzed sample with HCl. The crystal structure of 2 is
presented in Figure 1. A similar reaction occurs between 9,9-
Figure 1. Thermal-ellipsoid diagram of 2. Selected bond lengths and
angles: Si(1)-C(1) 1.8812(17), C(1)-C(2) 1.518(2), Si(1)-C(4)
1.8738(16), C(2)-C(2a) 1.337(3); C(1a)1-Si(1)-C(1) 95.25(11), C(4a)Si(1)-C(1) 120.45(7).
Angew. Chem. Int. Ed. 2006, 45, 2578 –2581
reaction occurred, however, with phenyldimethylsilane, a
typical silylene-scavenger reagent. In contrast, a fast reaction
of silylene 6 with ethyldimethylsilane is reported.[16] This
observation suggests that 6 is not an intermediate in the
reaction of 1 with DMB. Instead, we propose, on the basis of
DFT calculations,[17, 18] the mechanism which is shown in
Scheme 3 for the model reaction of compound 7 with 1,3butadiene.
Our starting point is the experimental observation that 1
exists in toluene as a THF solvate. Comparison of experimental 29Si NMR data for 1 with computed NMR chemical
shifts for the model compounds 7 and 8 indicates that, in
solutions of aromatic hydrocarbons, 1 adopts an h5–h5
structure, which is similar to the computed structure of
7[19, 20] (29Si NMR shift of 1 in [D6]benzene: d = 53.1 ppm;
computed: d = 58.8 (7), 39.3 ppm (8)).[21] After formation of
the encounter complex 9, the dilithiosilole 7 undergoes a
nucleophilic addition to butadiene, which is assisted by
complexation of the diene to the Li cation, thus yielding the
monoalkylated silole 10. Attack of the negatively charged dcarbon atom on the silicon atom completes the 1,4-addition
sequence with the formation of the reduced spiro compound
11. The formation of compound 11 is exergonic by 16.3 kcal
mol 1. Finally, 11 collapses by loss of elemental lithium and
dimethyl ether to yield the final product 12. The reaction
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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Communications
Scheme 3. Proposed mechanism for the cycloaddition of 1,3-butadiene
to 7.
profile of the proposed reaction mechanism, along with
computed structures of the intermediates 10 and 11 as well as
the transition states (TS) TSACHTUNGRE(9/10) and TSACHTUNGRE(10/11), is given in
Figure 2. The free-enthalpy barrier for the first step (DG298 =
16.6 kcal mol 1) is rate-determining, and the height of the
barrier is in qualitative agreement with a reaction which
proceeds smoothly at room temperature. The barrier for the
second step is only 2.4 kcal mol 1; therefore, an intermediate
monoalkylated silole similar to 10 would be hard to observe
experimentally. The transformation of 11 to the final products
12, Me2O, and Li0 would be strongly endergonic if all the
products remained in solution. In the actual reactions, the
transformation may be driven by the precipitation of lithium,
thus removing it from equilibrium.
The above reactions were carried out in toluene. When
the reaction of 1 with DMB was carried out in THF, the yield
of spirosilane 2 (ca. 25%) was much lower. This result may
have come about because the excess THF solvated the lithium
cations and removed them from the diene, thus making the
reaction with the double bond less favorable. The reaction of
disodium tetraphenylsilole with DMB also gave spirosilane 2,
but again in much lower yield (ca. 20 %). This result indicates
the importance of the size and the coordination ability of the
alkali-metal cation for the cyclization reaction, which emerges
also from the highly oriented transition-state geometries
(Figure 2), as predicted by the computations.
The scope of this new “oxidative-cyclization” reaction will
be further explored in our laboratories. Preliminary results
indicate that reactions take place between 1 and acetylenes
and diynes, but are much more complex.
Experimental Section
All procedures with air- and moisture-sensitive compounds were
carried out by using a Schlenk line under nitrogen or argon. 2,3Dimethyl-1,3-butadiene (DMB) and 1,3-cyclohexadiene were purchased from Aldrich and used without further purification. Solvents
were distilled from sodium benzophenone ketyl. NMR spectra were
recorded at room temperature on a Varian INOVA or Varian Unity500 spectrometer. The chemical shifts are expressed in ppm relative to
TMS as an internal standard.
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Figure 2. a) Calculated reaction coordinate for the reaction of 7 with
1,3-butadiene.[18] b) Optimized geometries of compounds TSACHTUNGRE(9/10), 10,
TSACHTUNGRE(10/11), and 11 (bond lengths are given in pm).
X-ray data collection and structural refinement for 2: Intensity
data were collected by using a Bruker SMART CCD area detector,[22]
mounted on a Bruker Platform goniometer, with graphite-monochromated MoKa radiation (l = 0.71073 F). The samples were cooled
to 133(2) K. For 2, the monoclinic space group P21/n (no. 14) was
determined by systematic absences and statistical tests, and verified
by subsequent refinement. The structure was solved by direct
methods and refined by full-matrix least-squares methods on F2. Xray data collection and structural refinement for 5 are presented in
the Supporting Information. CCDC-284150 and CCDC-284149 contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
2: Compound 1 was prepared from 1,1-dichloro-2,3,4,5-tetraphenylsilole (0.5 g, 2.2 mmol) and Li (0.033 g, 4.8 mmol) in THF (7.5 mL)
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2578 –2581
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Chemie
under argon as described previously.[3] The THF was removed by
vacuum and replaced with toluene. A solution of DMB (0.18 g,
2.2 mmol) in toluene (10 mL) was added to the solution of 1
(2.2 mmol) in toluene (10 mL) at 78 8C. After warming the mixture
to room temperature over 3 h, the reaction mixture was hydrolyzed
with a saturated solution of aqueous ammonium chloride. The organic
layer was extracted into diethyl ether, separated from the aqueous
layer, and dried over MgSO4. The solvents were evaporated in vacuo,
and the residue was crystallized from hexane/diethyl ether. Yield:
95 %. 1H NMR (500.0 MHz, C6D6): d = 1.6 (s, 6 H, CH3), 2.2 (s, 4 H,
CH2), 7.13–7.48 ppm (br m, 20 H, Ar); 13C NMR (125.75 MHz, C6D6):
d = 19.4 (s, CH3), 29.5 (s, CH2), 121.33–147.30 ppm (br m, Ar);
29
Si NMR (99.38 MHz, C6D6): d = 15.2 ppm. The reaction of 1 with
DMB in THF was performed by a similar procedure.
3: The reaction of 1 with 1,3-cyclohexadiene was performed as
described above for the experiment of 1 with DMB. Yield: 25%.
1
H NMR (500.0 MHz, C6D6): d = 1.59 (s, 4 H, CH2), 2.55 (s, 2 H, CH),
6.84–7.16 ppm (br m, 20 H, Ar); 13C NMR (125.75 MHz, C6D6): d =
20.40 (s, CH), 36.26 (s, CH2), 124.63–149.95 ppm (br m, Ar); 29Si NMR
(99.38 MHz, C6D6): d = 4.00 ppm.
Reaction of 1,1-disodium-2,3,4,5-tetraphenylsilole with DMB:
1,1-disodium-2,3,4,5-tetraphenylsilole was prepared as in ref. [1]. The
reaction was conducted as described above for 1 and DMB in toluene.
A complicated mixture of unidentified products was observed in the
reaction. 2 (20 %) was detected by 1H NMR spectroscopy.
5: Compound 4 was prepared from 9,9-dichloro-9-silafluorene
and lithium as described by Liu et al.[7] A solution of DMB (2.2 mmol)
in toluene (10 mL) was added at 78 8C to a solution of 4 (2.2 mmol)
in toluene (10 mL). The reaction mixture was worked up as described
above for 2. The product was crystallized from hexane/diethyl ether.
Yield: 87 %. 1H NMR (500.0 MHz, C6D6): d = 0.81 (s, 4 H, CH2), 1.82
(s, 6 H, CH3), 6.64–6.85 ppm (br m, 8 H, Ar); 13C NMR (125.75 MHz,
C6D6): d = 21.41 (s, CH3), 28.56 (s, CH2), 121.35–150.53 ppm (br m,
Ar); 29Si NMR (99.38 MHz, C6D6): d = 6.54 ppm.
Received: September 21, 2005
Revised: January 5, 2006
Published online: March 14, 2006
.
Keywords: reduction · silicon · silole dianions ·
spiro compounds
[11] T. Sanji, T. Sakai, C. Kabuto, H. Sakurai, J. Am. Chem. Soc. 1998,
120, 4552 – 4553.
[12] I. S. Toulokhonova, T. C. Stringfellow, S. A. Ivanov, A. Masunov,
R. West, J. Am. Chem. Soc. 2003, 125, 5767 – 5773.
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[14] Y. Liu, D. Ballweg, R. West, Organometallics 2001, 20, 5769 –
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[17] All calculations were carried out by using Gaussian 03, Gaussian
Inc., Pittsburgh, 2003.
[18] The structures were optimized by using DFT calculations at the
B3LYP/6-31G(d) level. Refined energies were obtained at the
B3LYP/6-311 + GACHTUNGRE(2d,p) level, and corrections for the free
enthalpy G at 298 K were computed at the B3LYP/6-31G(d)
level. These corrections were added to the energies obtained at
the B3LYP/6-311 + GACHTUNGRE(2d,p) level. Each stationary point was
identified either as an intermediate or transition state (TS) by a
subsequent frequency calculation. Intrinsic reaction coordinate
(IRC) computations were used to connect the transition states
with the appropriate minima. The influence of the basis-set size
on the computed geometries and relative energies was tested at
the B3LYP/6-311 + GACHTUNGRE(d,p) level of theory for several compounds and was found to be insignificant in all cases. All
calculated data are presented in the Supporting Information.
[19] The dilithium salt 1 crystallizes from THF in the h1–h5 form (see
ref. [3]); in solution, however, the h5–h5 isomer prevails (see
ref. [20]).
[20] T. MLller, Y. Apeloig, H. Sohn, R. West in Organosilicon
Chemistry III: From Molecules to Materials (Eds.: N. Auner, J.
Weis), Wiley-VCH, Weinheim, 1998, pp. 144 – 151.
[21] Computed at GIAO/B3LYP/6-311 + GACHTUNGRE(2d,p)//B3LYP/6-31G(d).
Calculated s(Si) for TMS: 329.3.
[22] Bruker-AXS. (2000–2003) SADABS V.2.05, SAINT V.6.22,
SHELXTL V.6.10 & SMART 5.622 Software Reference Manuals. Bruker-AXS, Madison, Wisconsin, USA.
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Angew. Chem. Int. Ed. 2006, 45, 2578 –2581
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www.angewandte.org
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